The mouse mammary gland is being studied as a model for radiation carcinogenesis and especially the effect of X-rays on hyperplastic lesions of the mammary gland. The initial experiments compare the effects of X-rays on the survival of normal, premalignant and malignant mammary tissues. Dose-response parameters were established for the various tissues using the ability of transplanted samples of irradiated tissues to grow. The initial experiments demonstrate that radiation effects can be detected in the mouse mammary system when samples of the irradiated mammary gland are transplanted to non-irradiated hosts. Increasing doses of radiation delayed the growth of many transplants, reduced the percentage of transplants showing any growth, and induced morphologic changes. For comparison, a second set of experiments was performed in which transplants of mammary tissue were irradiated in the fat pad. An exposure chamber for mice was fabricated which allowed the X-irradiation of the inguinal fat pad without exposing the rest of the mouse. Exposures of up to 1200 rad at the gland had less than one rad to the body of the mouse. It was estimated that 90-95% of the fat pad was exposed using this technique. Long-term studies have been started to study carcinogenesis of low levels of X-rays to normal and premalignant mammary tissues.